The use of tracers to monitor aspects of the performance of hydrocarbon wells is an established technique. The tracers may be water tracers, in that they are predominantly soluble in water, oil tracers, in that they are soluble in the hydrocarbons in the formation, or partitioning tracers, in that they move between the water and hydrocarbon or back. Some tracing methods will more than one type of tracer and use the difference in behaviour to deduce properties of the hydrocarbon formation. For example, partitioning and water tracers may be injected into a production well along with injected water and then monitored as they are subsequently produced from the well. The time difference between the production of the water tracers, which are produced with the returning injected water, and the partitioning tracers, whose production is delayed by their interaction with the hydrocarbons in the formation, can be used to deduce parameters relating to the local remaining hydrocarbon content of the formation. Alternatively, applications may use only water tracers. For example, water tracers may be introduced in an injection well and their presence monitored at adjacent production wells in order to obtain information about the flux of water from the injection well to the production well.
In addition to injected techniques, it is also known to introduce tracers into a well by including them in articles placed into the well. For example, the tracers may be mixed with a polymer and cast into an article that is inserted into the well when the well is constructed. The tracer is then eluted from the polymer over time as fluid flows past the article. By detecting the rate of tracer production over time, information can be deduced about production of water or oil in the reservoir.
Examples of tracer techniques are described in EP1277051 and U.S. Pat. No. 8,640,773.
Many tracing techniques measure a property of a region of a well or formation relative to the properties of surrounding regions of the well or formation. In order to do that, different tracers are introduced into the different regions, whether by injection, placement during well construction, or another method. The production of each of the different tracers can be monitored in samples produced from the well to obtain information about where the produced fluids have come from. In addition, tracing techniques may be used sequentially on wells that have previously been traced. As an example, an inter-well tracer study may be used to monitor injected water flux from an injection well to a production well and a later study may then inject tracers into the same injection well or a different well to monitor the levels of hydrocarbon remaining in the well. If the same tracer is used for two different regions, or in two different studies, the analysis of the produced sample may be contaminated by tracer from the wrong region or the previous study. In a typical (“conventional”) well, there may be demand for studies involving 10-20 different tracers, but for some applications, for example hydraulic fracturing applications, it may be desirable to monitor as many as 40 different zones per lateral bore, with several laterals in a well. There is therefore a need for new tracers and in particular a need for new families of tracers.
Fluorinated benzoic acid salts are often used as water tracers in hydrocarbon well monitoring. A number of possible tracer variants exist in the fluorinated benzoic acid family since the benzoic acid can be mono-, di-, tri-, tetra- or penta-fluorinated and the fluorination can, except for the penta-fluorinated case, be at various locations on the aromatic ring. Nevertheless, there are a finite number of variants of fluorinated benzoic acids.
In order to be useful as a tracer, a compound should be thermally stable in that it should be stable at the temperatures typically encountered in wells, which may be 60 to 90° C. Desirably, a tracer is stable in temperatures up to maybe 160 or 180° C. so as to permit use in high temperature wells. For a water tracer, the compound should be highly selective toward water over oil. The compound should also be detectable in very small quantities, preferably at levels of 10 ppb or lower and most preferably in the parts per trillion (ppt) range (that is, at levels less than 1 ppb). The levels are determined on a mass/mass basis. The compound should also be environmentally acceptable, for inserting into the ground, but also not a compound that is naturally present in the ground in such quantities as to contaminate the results of the tracer study.
Typical detection methods include gas chromatography mass spectrometry (GC-MS), gas chromatography mass spectrometry mass spectrometry (GC-MS-MS), liquid chromatography mass spectroscopy (LC-MS), liquid chromatography mass spectroscopy mass spectroscopy (LC-MS-MS) and high pressure liquid chromatography (HPLC), which can typically detect very low concentrations of the tracers in the produced fluids. It is desirable that tracers should be detectable in low quantities and also that they can be reliably distinguished from other tracers.
Further examples of tracers are disclosed in EP2563874.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide new tracer compounds for use in hydrocarbon well monitoring.